Field of the Invention
The present invention relates to an image forming apparatus, for example, a digital copying machine, a multifunctional peripheral, or a laser printer, which is configured to correct distortion and uneven image density of an image during image formation of a two-dimensional image by the image forming apparatus.
Description of the Related Art
In electrophotographic image forming apparatus such as a laser printer and a copying machine, there has been generally known a system in which a latent image is formed on a photosensitive member with use of a light scanning device configured to perform scanning with a laser beam. The light scanning device of a laser scanning type employs a scanning exposure system in which a laser beam collimated with use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member with use of an elongated fθ lens. In such a scanning exposure system, scanning timing of a light beam for scanning with the rotary polygon mirror is detected by a light detector (hereinafter referred to as “BD”), and writing timing of an image is controlled such that the image is written at a predetermined position on the photosensitive member. Through the scanning timing detection by the BD, the writing timing on the photosensitive member can be controlled based on the rotation speed and phase of the rotary polygon mirror.
However, the system configured to control the writing timing by the BD has a problem in that, due to an incorrect angle of mirror faces (reflecting faces) of the rotary polygon mirror or incorrect flatness of the mirror faces, printing positions in an image area differ between the respective mirror faces (this is referred to as an optical face tangle error of the rotary polygon mirror). Such positional deviation due to a tolerance of the mirror faces of the rotary polygon mirror repeatedly occurs every time the rotary polygon mirror rotates once, leading to periodic uneven image density. In order to solve the problem, for example, in Japanese Patent Application Laid-Open No. 2004-271691, there is disclosed a system configured to store a difference in scanning start positions between the respective mirror faces of a rotary polygon mirror to control emission timing.
In image forming apparatus as represented by laser printers, a relationship of a density control amount to a light amount changes when the image forming apparatus degrades due to a change in temperature environment or use. Thus, it is difficult to stably correct banding with use of a method such as the one in Japanese Patent Application Laid-Open No. 2004-271691 described above, which adjusts a density by adjusting an exposure amount because the density control amount tends to change. In addition, when positional deviation occurs for a relatively long period in a color image forming apparatus, positional deviation occurs between the colors of toners for a long period, leading to image defects including uneven hue. FIG. 15 is a diagram for illustrating positional deviation of each of scanning lines. A positional deviation amount in an image area between an ideal position of a scanning line and an actual scanning position varies in a non-uniform manner due to factors including fluctuation in surface speed of a photosensitive drum. A related-art image forming apparatus is not capable of moving an image position, and hence there is a problem in that uneven hue is not reduced.
In addition, there is a problem in that, when a temperature environment around the printer changes, the deviation amount of the scanning position of a light beam for scanning the photosensitive member changes. FIG. 16A is a diagram for illustrating scanning positions for each face of the rotary polygon mirror before and after a rise in temperature. As illustrated in FIG. 16A, there is a problem in that, even when the photosensitive member is positioned so as to match with a conjugate point in an initial state (e.g., adjustment at a factory), the conjugate point moves due to the rise in temperature to cause the positional deviation.